1. Field of the Invention
The present invention relates to a capacitance-type fingerprint sensor, and more particularly to a capacitance-type fingerprint sensor using a two-stage electrode.
2. Description of the Related Art
A fingerprint recognition system acquires a fingerprint image, and compares the acquired fingerprint image with a pre-stored fingerprint image using unique characteristics of people's fingerprints such that it can recognize a specific person from among a plurality of persons. The fingerprint system has been widely used for security and authentication systems, and includes a fingerprint sensor for acquiring a fingerprint image and an image processor for comparing the acquired fingerprint image with a pre-stored fingerprint image. Typically, the fingerprint sensor includes an image sensor and additional devices (e.g., a light source, and a lens, etc.) for generating an optical fingerprint image. There have recently been researched semiconductor-type sensors capable of acquiring a fingerprint using only a single chip without using additional devices required for a conventional optical system, such that they have been widely applied to mobile devices such as a mobile phone and a smart card, etc.
Particularly, the capacitance-type fingerprint sensor from among the above-mentioned semiconductor-type sensors is adapted to recognize a fingerprint using a difference in capacitor's capacitance generated when the fingerprint contacts a sensing electrode, and has an advantage in that it can be implemented with a conventional Complementary Metal Oxide Semiconductor (CMOS) circuit fabrication without using additional processes, such that it is most widely used to recognize a fingerprint.
A basic capacitance-type fingerprint sensor structure has been described in U.S. Pat. No. 4,353,056 filed by Siemens Corporation in Germany, entitled “CAPACITIVE FINGERPRINT SENSOR”. A structure for enhancing an output fingerprint image quality of a fingerprint sensor and detecting characteristics of the output fingerprint image has been described in IEEE Journal of Solid-State Circuits, 1999, vol. 34, no. 7, pp. 978˜984, proposed by S. Jung. A method for removing a parasitic component from a capacitance-type fingerprint sensor and enhancing a fingerprint image quality of the fingerprint sensor has been described in IEEE International Solid-State Circuit Conference on 2002, proposed by K. Lee.
FIGS. 1a˜1c are schematic diagrams illustrating a sensing unit of a conventional capacitance-type fingerprint sensing apparatus.
Referring to FIG. 1a, the conventional capacitance-type fingerprint sensing apparatus detects capacitor's capacitance formed between a finger skin surface and each uppermost metal plate 10 used as a sensing electrode, such that it acquires a fingerprint image. The metal plate 10 acting as a sensing electrode has a two-dimensional layout, and each pixel positioned at a lower part of the metal plate 10 includes a detection circuit 20 for detecting capacitor's capacitance. An output terminal of the detection circuit 20 contained in each pixel is connected to a column-direction selection drive 22 by a row-direction selection drive 21, and output signals of pixels arranged by the column-direction selection drive 22 are sequentially generated.
Referring to FIG. 1b, the detection circuit 20 connected to the metal plate 10 acting as a sensing electrode is formed in an insulating layer 40 formed on a substrate 30, such that it is insulated from an external part. However, not only capacitance formed between a finger and a sensing electrode to form a fingerprint image, but also parasitic capacitance is formed among the sensing electrode, the substrate and the detection circuit, such that the parasitic capacitance acts as a fixed signal irrespective of the fingerprint image.
FIG. 1c is a schematic diagram illustrating how a fingerprint image is differently represented according to the degree of moisture left on a finger surface in the case of using the fingerprint sensing apparatus shown in FIGS. 1a˜1b. 
Referring to FIG. 1c, if the finger surface is not in an especially dry or moist state, the skin of the finger acts as an electrode, such that a fingerprint image matched with a real fingerprint is formed. Otherwise, if the finger surface is moist, moisture left on the finger surface acts as an electrode. Also, if the finger surface is dry, the dermis of the finger acts as an electrode, such that a fingerprint image different from a real fingerprint is formed.
As mentioned above, capacitor's capacitance formed when the sensing electrode contacts the finger surface has difficulty in acquiring a high-quality fingerprint image due to parasitic capacitance formed in a circuit capable of detecting the dry or moist state of a finger surface or the capacitor's capacitance, such that it causes the conventional fingerprint recognition system to be erroneously operated.